1. Field of the Invention
The present invention relates to directional couplers, and in particular, relates to a directional coupler that only couples microwaves that propagate through a transmission line in a first direction and obtains an output proportional to the microwave power and that does not couple microwaves that propagate through the transmission line in a second direction opposite to the first direction.
2. Description of the Related Art
For example, as described in Japanese Unexamined Patent Application Publication No. 5-160614, waveguide circuits, which have been the predominant microwave circuits, require high precision machining and thus are not suitable for mass production and are expensive. Moreover, a problem has existed with waveguide circuits in that the outer dimensions and weights of the waveguide circuits are large. Thus, microstrips, which can be reduced in size and weight through the use of large-scale integration technology have been used in radios, BS receivers, and the like.
A conventional directional coupler composed of microstrips shown in FIG. 6 is disclosed in Japanese Unexamined Patent Application Publication No. 5-160614.
This directional coupler is a side-edge type coupler, which has a structure in which sections of respective stripline electrodes 81a and 82a of microstrips 81 and 82 are disposed close to each other for the length of λ/4 in the horizontal direction and the upper and lower surfaces of the microstrips 81 and 82 are covered with ground electrodes 83 and 84. In a coupled mode of the sections of the stripline electrodes 81a and 82a which are disposed close to each other, a first microwave power is input from a port 1 to the microstrip 81 functioning as a main line while a second microwave power, that is a fraction of the first microwave power, is generated in a port 3 of the microstrip 82 functioning as a subordinate line.
For example, as shown in FIG. 7, in a cellular phone unit, in order to keep the transmission power at a minimum level through the function of dividing high frequency signals into two components in the aforementioned directional coupler, a main line 70a of a directional coupler 70 is disposed between a transmission power amplifier 71 and an antenna 72 and one end of a subordinate line 70b is connected to an automatic gain control circuit 73 so that the automatic gain control circuit 73 adjusts the output of the transmission power amplifier 71.
With regard to cellular phone units and the like, an important issue is to minimize the size. Thus, the size of directional couplers has been required to be further reduced. However, in the directional coupler shown in FIG. 6, for example, λ/4 is 7.5 cm (on the condition that the specific inductive capacity is 1) at 1 GHz. Thus, the required minimum length of the sections disposed close to each other in the horizontal direction of the stripline electrodes 81a and 82a is 7.5 cm. Accordingly, the size of the substrate, which includes the stripline electrodes 81a and 82a thereon, becomes large. Moreover, for example, when respective substrates that include the ground electrodes 83 and 84 thereon are disposed and fastened with screws under and over the substrate, which includes the stripline electrodes 81a and 82a thereon, a problem arises in that a reduction in size is limited and the cost increases.
Accordingly, a directional coupler that is improved to solve the aforementioned problem is proposed in Japanese Unexamined Patent Application Publication No. 5-160614. In this directional coupler, ground electrode substrates that include ground electrodes thereon are alternately laminated with dielectric substrates on which a pair of stripline electrodes are provided so that the stripline electrodes are disposed close and parallel to each other in a spiral shape. Then, the corresponding stripline electrode components of the individual dielectric substrates are connected in series with each other through a pair of via holes that are close to each other so that the stripline electrodes have the length of a quarter of a wavelength.
In the improved directional coupler, the stripline electrodes having the length of a quarter of a wavelength are formed with the stripline electrode components and the via holes so that the stripline electrodes are divided into components on a plurality of laminated dielectric substrates. Thus, the size of the improved directional coupler can be small compared with that of the directional coupler shown in FIG. 6. However, even in the improved directional coupler, the total length of the stripline electrodes is required to be a quarter of a wavelength. Thus, the size of the directional coupler cannot be significantly reduced. Moreover, in general, side-edge type couplers have a problem in that it is difficult to achieve a high degree of coupling due to the characteristics of the distribution of a magnetic field around the stripline electrodes. The improved directional coupler uses side-edge coupling between a pair of stripline electrodes. Thus, the improved directional coupler has a problem in that it is difficult to achieve a high degree of coupling.
On the other hand, a directional coupler called a broad-side type coupler is proposed in Japanese Patent No. 3203253. In this directional coupler, spiral-shaped coupled lines are opposed to each other with dielectric layers therebetween so as to achieve coupling between the coupled lines. Since the inductance value of the coupled lines becomes high in the directional coupler, the directional coupler can be constructed with lines that are shorter than a quarter of a wavelength. Thus, the size can be readily reduced, and a high degree of coupling can be achieved with a small loss.
However, in the directional coupler disclosed in Japanese Patent No. 3203253, since spiral-shaped coupled lines are opposed to each other with dielectric layers therebetween so as to achieve coupling between the coupled lines, the capacitance between the coupled lines becomes large. Thus, the directional coupler has a problem in that high isolation between the coupled lines cannot be achieved.
Moreover, in the directional couplers disclosed in Japanese Unexamined Patent Application Publication No. 5-160614 and Japanese Patent No. 3203253, coupling is adjusted by adjusting the distance between lines. In this case, a magnetic field and an electric field around the lines are both changed by adjusting the distance between the lines, and it is impossible to adjust only one of the magnetic field and the electric field. Thus, it is difficult to adjust isolation. Isolation is a phenomenon in which magnetic field coupling and electric field coupling nullify each other. Thus, isolation has been adjusted only by selecting types of materials of substrates on which coupled lines are provided to change the permittivity and the permeability.